The present invention relates to a magnetic recording medium capable of recording a great amount of information and, more in particular, it relates to a magnetic recording medium suitable for high-density magnetic recording.
There has been an increasing demand for magnetic storage apparatus typically represented by magnetic disk apparatus with a greater capacity. In order to cope with the demand, a development has been required for a magnetic head with high sensitivity and a recording medium capable of obtaining high signal output-to-noise ratio (S/N).
Generally, a recording medium comprises a first underlayer referred to as a seed layer formed over a substrate, a second underlayer of a body-centered cubic structure comprising a chromium alloy, a magnetic layer and a protective layer comprising carbon as a main component. An alloy having a hexagonal close-packed structure comprising cobalt as a main component is used for the magnetic layer.
To improve S/N, it is effective to crystallographically orient a magnetic layer such that the (11.0) plane or (10.0) plane is substantially in parallel with the surface of a substrate and direct the c-axis of the hexagonal close packed structure as an axis of easy magnetization in the direction within the plane of the film. It is known that the crystallographic orientation of the magnetic layer can be controlled by the seed layer and such orientation can be obtained by using tantalum (Ta) or W—Co alloy, etc. for the seed layer. Further, it has been known that the magnetic characteristic in the circumferential direction can be improved by applying mechanical texturing to the surface of the substrate and introducing magnetic anisotropy in the circumferential direction.
For the improvement of S/N, it is effective to adopt a multi-layered structure for the magnetic layer, refine the crystal grain size or decrease Brt which is a product of the residual magnetic flux density Br and the film thickness (t) of a magnetic layer. Patent Document 1 (Japanese Patent Laid-open No. 2001-56923) proposes a magnetic recording medium in which an underlayer is disposed on a substrate. The document describes that a stacked magnetic film comprising at least two magnetic layers of different compositions in contact with each other is disposed on the underlayer as a multi-layered structure by way of a non-magnetic layer such as one made of ruthenium. It further proposes a magnetic recording medium including an underlayer over a substrate and a magnetic recording layer formed thereabove in which the magnetic recording layer has a multi-layered structure vertically separated by an intermediate layer. The intermediate layer is formed of one of materials selected from the group consisting of Ru, Rh, Ir, and alloys thereof selected from the range of from 0.2 to 0.4 nm and from 1.0 to 1.7 nm, and the magnetization directions of the magnetic recording layers separated vertically by the intermediate layer are in parallel with each other. Use of the magnetic recording medium described above provides a magnetic recording medium having a magnetic recording layer of a multi-layered structure intended for reducing noise while maintaining magnetic characteristic having thermal stability.
Since an extreme miniaturization of the crystal grains used for the magnetic recording layer or large reduction of the Brt results in the degradation of the thermal stability, the reduction of noise is limited. In recent years, an anti-ferromagnetically coupled (AFC) medium to be described later has been proposed as a technique capable of compatibilizing the thermal stability and the noise reduction. This is a dual layered structure of two anti-ferromagnetically coupled magnetic layers by way of an Ru intermediate layer which can set the Brt lower while maintaining the thickness of the large ferromagnetic film as it is, compared with a medium comprising a single layered magnetic layer. Accordingly, this could reduce the medium noise while maintaining the thermal stability.
Further, with an aim of providing a magnetic recording medium which is favorable in the thermal fluctuation characteristic (thermal stability) and favorable in the read/write characteristic such as coercivity squareness (S), pulse width, overwrite characteristic, medium noise (S/N ratio), etc. Patent Document 2, (Japanese Patent Laid-open No. 2004-355716) for example, proposes a magnetic recording medium. This magnetic recording medium comprises an underlayer, a first magnetic layer, a first intermediate layer, a second magnetic layer, a second intermediate layer, a third magnetic layer, a protective layer, and a lubricant layer which are formed in this order above the substrate. In addition, each of the third magnetic layer and the second magnetic layer comprises a cobalt (Co)-based alloy containing at least platinum (Pt), chromium (Cr), and boron (B). The concentration of platinum contained in the second magnetic layer is less than the concentration of platinum contained in the third magnetic layer and, at the same time, platinum contained in the third magnetic layer is 15 at. % or less. The concentration of chromium contained in the third magnetic layer is 15 at. % or more and 18 at. % or less, and the concentration of boron contained in the third magnetic layer is 7 at. % or more and 10 at. % or less.